In recent years, the number of patients suffering from respiratory diseases such as asthma, pulmonary emphysema, chronic bronchitis and the like has tended to increase. One of the most effective therapeutic methods for these diseases is oxygen inhalation therapy and oxygen concentrators which separate oxygen-concentrated gas directly from an oxygen tank or air have been developed and used as therapeutic apparatus for oxygen inhalation therapy.
As the oxygen concentrator, a pressure swing adsorption-type (hereinbelow refer to as PSA-type) oxygen concentrator, which produces a highly concentrated oxygen gas by introducing compressed air from a compressor into an adsorption cylinder filled with an adsorbent such as zeolites and the like selectively adsorbing a nitrogen gas to eliminate selectively a nitrogen gas in the air, has widely been used.
The PSA-type oxygen concentrator produces a highly concentrated oxygen gas from air through the steps described below. Firstly, compressed air is supplied by a compressor into an adsorption cylinder to adsorb a nitrogen gas contained in air by an adsorbent such as zeolites and the like. The oxygen concentration in air is increased by selective adsorption and elimination of a nitrogen gas. The highly concentrated oxygen gas thus obtained is transferred to a buffer (product tank) storing oxygen gas via a check valve which is installed to prevent the gas from flowing back and is stored therein.
At the same time, a nitrogen gas adsorbed by the adsorbent is desorbed from the adsorbent by reducing the inner pressure of the adsorption cylinder to recover the efficiency of the adsorbent. A small amount of concentrated oxygen gas may also be supplied into the adsorption cylinder to promote the desorption of nitrogen.
Thus, highly concentrated oxygen-enriched air having an oxygen concentration as high as 95% may be obtained from air by repeatedly pressurizing and depressurizing the adsorption cylinder using the PSA-type oxygen concentrator. In recent years, as such a PSA-type oxygen concentrator, a multiple cylinder-type oxygen concentrator has been used, which is equipped with multiple adsorption cylinders filled with an adsorbent selectively adsorbing a nitrogen gas and in which oxygen concentration efficiency is increased by successively switching these adsorption cylinders by a rotary valve.
It has been known that an oxygen concentration obtained largely depends on the pressure of air to be compressed by a compressor in the PSA-type oxygen concentrator. It is thus required, in order to obtain a highly concentrated oxygen gas, to increase the capacity of the compressor to meet an increase in the pressure of air to be supplied to the adsorption cylinder as far as possible. A problem arises, however, that power consumption of apparatus increases with the elevation of the capacity of the compressor to supply compressed air.
Various attempts had been made as measures for solving these problems of the PSA-type oxygen concentrator. Patent Document 2 discloses a PSA-type oxygen concentrator which realizes reduction in electric power consumption of the apparatus by changing the supply capacity of a compressed air supply device such as a compressor and the like based on a value detected by an oxygen concentration detector for oxygen gas to control oxygen concentrations at a constant value.                Patent Document 1: Japanese Patent Laid-open Publication No. 2006-141896        Patent Document 2: Japanese Patent Laid-open Publication No. 2007-000340        Patent Document 3: Japanese Patent Laid-open Publication No. 2002-253675        